Transmission line thermal short



Oct. 21, 1969 7 JAMES E. WEBB 3,

ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONTRANSMISSION ,LINE THERMAL SHORT Filed Sept. 15, 1967 ggggg FIGQI F|G.2

x 52m 68 De I E 54 DD I V 16 I 4 re MICROWAVE EN SIGNALS ATTORNEYSUnited States Patent 3,474,357 TRANSMISSION INE THERMAL SHORT James E.Webb, Administrator of the National Aeronautics and SpaceAdministration, with respect to an invention of Robert C. Clauss, LaCrescenta, Calif.

Filed Sept. 15, 1967, Ser. No. 668,247

Int. Cl. H01p 3/06 US. Cl. 33396 18 Claims ABSTRACT OF THE DISCLOSUREORIGIN OF THE INVENTION The invention described herein was made in theperformance of work under a NASA contract and is subject to theprovisions of Section 305 of the National Aeronautics and Space Act of1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457) BACKGROUND OF THEINVENTION This invention relates to electromagnetic wave transmissiondevices and more particularly to means for cooling components thereof.

The detection of very low level signals is often accomplished withamplifiers that must be cooled to very low temperatures. For example,communication with space probes at distances of millions of miles hasbeen accomplished by feeding the signal from an antenna to a travelingwave maser operated at a temperature of 4.4 K. The maser amplifier andits refrigerator are mounted on a movable antenna structure, and boththerefore must be compact and light weight. Additionally, such antennasare often situated at remote locations to decrease interference, andrepair and replacement of parts must be as simple as possible.

The signals received by a tracking station antenna are typicallydirected to a waveguide horn and carried by a wave-guide to the maseramplifier. The frequency used, such as 2.3 K. mHz. necessitates arelatively large waveguide, such as several inches in width. To extendsuch a waveguide to the cryogenically cooled maser amplifier wouldresult in very large thermal paths and require excessively largerefrigeration apparatus. Accordingly, at the entrance to the maserrefrigeration apparatus, the waveguide is connected to a coaxial cablewhich conducts a signal to the maser. The coaxial cable allows arelatively small line to be used; however, due to resistive losses, itadds substantial noise. Furthermore, the coaxial cable conducts heat tothe maser and adds considerable refrigeration load. The refrigerationwhich is required would be inefficiently utilized if all cooling wereapplied from the 4.4 K. refrigeration point at the maser apparatus.

Both the noise contributed by the coaxial cable and the refrigerationload can be reduced by cooling the coaxial cable along its centerportion to a temperature in between ambient temperature (nominally 290K.) and the maser operating temperature of 4.4 K. The outer conductor ofthe cable is cooled by applying a refrigeration stage which is at amoderately low temperature, directly to the outer conductor. However,considerable difliculty is encountered in cooling the inner or centerconductor, since it is electrically insulated from the outer conductor.Inasmuch as materials which are good thermal conductors are alsogenerally good electrical conductors, the direct connection of a goodthermal conductor such as a metal between the inner and outer conductorswould result in an electrical connection between them. This would resultin a high voltage standing wave ratio (VSWR). A high VSWR would preventsome signal power from reaching the maser amplifier, thereby decreasingsignal-to-noise ratio. This invention provides a means for conductingheat from the coaxial cable center conductor to the outer conductor witha minimum increase in VSWR over a considerable bandwidth, and withoutappreciably changing the impedance of the coaxial line, thereby enablingcooling of a line at a center portion thereof to decrease noise anddecrease the 4.4 K. refrigeration load.

SUMMARY OF THE INVENTION The present invention provides a thermal shortcircuit between the outer and center conductors of a coaxialtransmission line without substantial deleterious effects, byintroducing conductive bars or wires between the center and outerconductors. The bars extend approximately a quarter wavelength along thelength of the coaxial cable, and have one end in contact with the outerconductor. In addition, the center conductor is provided with a portionof slightly greater diameter, extending along most of the length of thewires. The conductive bars are of a material which normally conductsboth heat and electricity. In the present invention, heat is conductedfrom the center coaxial conductor through the bars to the outerconductor. However, the microwave signal power is not shorted at thispoint because the conductive bars extend approximately a quarterwavelength before contacting the outer conductor, and the VSWR has onlya small increase. An enlarged portion of the center conductor serves tomaintain the proper impedance characteristics of the coaxial line sothat there is a good match between it and the waveguide output and maserinput to which it is connected.

In the particular embodiment of the invention illustrated herein, theconductive bars are soldered to the center conductor and make thermalcontact with the outer conductor through contact fingers. The contactfingers are formed in a thin cylinder of metal having a diameterslightly smaller than the inner diameter of the outer coaxial conductor,thereby assuring large areas of contact. The thermal short circuit ofthe invention does not increase the bulk of the coaxial line. The centerconductor of the coaxial line can be removed and replaced in a simplemanner, thereby facilitating both initial assembly and replacement inthe field.

A more complete understanding of the invention can be had by consideringthe following detailed description of a preferred embodiment, theclaims, and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a side sectional view of aportion of an amplifier system employing the invention;

FIGURE 2 is a partially sectional side elevation view of the invention;I

FIGURE 3 is a sectional view of the invention; and

FIGURE 4 is a view taken on the section 44 of FIGURE 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGURE 1 illustrates a portionof the input line to a traveling wave maser for amplifying very lowlevel microwave signals. The signals are initially received through awaveguide which is connected to a feed horn of an antenna directed atthe signal source, which may be a transmitter on a distant space probe.The signals, which are generally very weak, pass through a coaxial cable12, or coaxial transmission line, to the input 14 of a traveling wavemaser amplifier. The coaxial cable 12 is located in a refrigeratorstructure 16 which has a protective and insulating housing 18 around it,and which maintains a vacuum within the housing to provide insulation.The refrigerator structure has several refrigeration stations within it.A first, which maintains a moderately low temperature such as 80 K., isattached to first refrigeration plate 20. A final station, whichmaintains a temperature close to absolute zero, such as 4.4 K., isconnected to final refrigeration plate 22. The final refrigeration plate22 contacts the maser, which normally operates at temperatures below 4.5K.

The coaxial line 12 has an outer conductor 24 and an inner or centerconductor 26 for carrying the micro waves from the waveguide to themaser. The outer conductor 24 of the coaxial line is connected to thewaveguide 10 by a holding flange 28. The center conductor 26 isconnected to the waveguide by a center pin or bullet 3.0 which isinserted into a receiving hole in a Waveguide to coaxial line transition32, which extends between the walls of the waveguide and its center. ATeflon washer 36 helps to separate the center and outer coaxialconductors and maintain alignment. A flange 38 on the outer conductor 24provides a firm attachment of the coaxial line to the refrigeratorhousing 18 and seals the covering around the hole through which thecoaxial line enters.

The inside of the refrigerator structure 16 is maintained at a vacuum toreduce heat transfer. A vacuum seal 40 between the center and outerconductors of the coaxial line seals the line, so that the space betweenthe center and outer conductors is maintained at a vacuum. O-ring 42 onthe vacuum seal and O-ring 44 on t the flange 38 help to maintain thevacuum seals.

At the end of the coaxial cable 12 which connects to the maser input 14,connections are provided by a tapered end portion 46 of the centerconductor, having a split sleeve which receives a pin 48 of one of themaser input lines 14 which is a tubular copper coaxial line. The outerconductor 24 of the coaxial line is physically connected to the finalrefrigeration plate 22, which is connected to the maser.

Thermal insulation between the waveguide 10, which is at ambienttemperature, and the near absolute zero temperature of the finalrefrigeration plate 22 is accomplished by the insulating properties ofthe vacuum within the refrigeration structure 16, and by constructingthe center and outer conductors 26 and 24 of the coaxial line ofrelatively poor thermally conducting material, such as stainless steel.In order to provide a low loss path for microwaves through the coaxialline 12, the inner surface of the outer conductor 24 and the outersurface of the inner conductor 26 are plated with copper; a gold flashlayer is deposited over the copper to prevent corrosion. Inasmuch asvery high frequency waves pass through the coaxial line 12, theelectrical currents flowing through the center and outer conductors havea very small skin depth (approximately 0.000050 inch at 2,300 mHz.) anda very thin layer of copper is sufficient. Inasmuch as the stainlesssteel tubing of the center and outer conductors is relatively thin andof relatively long length, the amount of heat leaked therethrough isvery small.

It should be noted that the thermal gradients, or variations intemperature, along the center conductor 26 are not linear because thethermal conductivity of stainless steel and of copper varies as afunction of temperature, the conductivity of stainless steel being lowerat lower temperatures. As a result, the refrigeration load is reduced bycooling the center conductor 26 at the first refrigeration station 20,instead of only at the final station 22. Furthermore, refrigerationapparatus is more efficient when the cooling is not to an extremely lowtemperature, such as that of the final station 22, and the cooling atfirst station 20 reduces the load at the final station. In one deviceconstructed according to the embodiment of FIGURE 1, the heat leakthrough the center portion of the coaxial line was reduced from 75milliwatts to 25 milliwatts by the addition of a thermal short.

Although reduction of refrigeration load is helpful, an even moreimportant benefit from cooling the center conductor at an intermediatepoint, in many cases, is reduction of microwave noise. Noise arisesbecause of the high temperature of the coaxial line, and cooling theline over as much of its length as possible reduces this noiseconsiderably. In the device constructed in accordance with FIGURE 1,noise generated in the cooled portion of coaxial line, between thethermal short and the maser input, was reduced to one-ninth its previouslevel by cooling the center conductor from approximately 290 K. to K.near its beginning.

The present invention is directed primarily to the cooling of the inneror center conductor 26 of the coaxial line. The difiiculty of coolingthe center conductor is due to the fact that it must be electricallyinsulated from the outer conductor 24, at least insofar as it concernsmicrowave power being carried. The thermal conduction between the centerand outer conductors occurs through shorting bars 52 and 54 and acylinder 56 which extend between the center and outer conductors. Thecylinder 56 has finger 58 which contact the outer conductor 24 at apoint along its length which is adjacent to the first refrigeratingplate 20. Placement of the fingers 58 adjacent to the first plate 20reduces the resistance of the thermal path between the outer and centerconductors. If the heat had to be transmitted through a considerablelength of the outer conductor 24-, less heat would betransmitted fromthe center conductor. As discussed above, an important reason forcooling the inner conductor 26 along its length is to increase itsconductivity and reduce the amount of noise added by the coaxial line.This occurs between an area slightly above the point 60, where coolingbegins, and the final refrigerator plate 22. As has been also mentioned,the cooling at the point 60 reduces the thermal conductivity of thecenter conductor between an area near that point and the finalrefrigerator plate 22 so that even less heat is transmitted to the finalplate. The particular construction of the thermal short 51 is adapted toefficiently transmit heat from the center conductor to the first stageof refrigeration while providing a minimum of disruption in electricaltransmission characteristics for the microwave frequency at which thecoaxial line is to be used.

A better view of the thermal short 51 is provided in FIGURES 2 and 3.The thermal short is constructed with a conductor rod 62 having aplug-like end 64, a center portion 66, and an expanded portion 68 whichincludes a sleeve-like end 70. The conductor rod 62 is preferablyconstructed out of a single piece of copper, to reduce the number ofjoints which might act as thermal insulators. The plug-like end 64 hasan outer diameter approximately equal to the inner diameter of theconductor 26, and is soldered in place to provide mechanical strengthand good heat transfer characteristics. The center portion 66 has adiameter equal to that of the center conductor 26. The expanded portion68 has a diameter larger than the diameter of the center portion 66 by aclosely controlled amount, and the axial extension D of the expandedportion is also closely controlled. A hole in the expanded portion 68extending from the end 70 inwardly, has a diameter approximately equalto the outer diameter of the center conductor 26, and after the centerconductor is inserted into the conductor rod 62, they are solderedtogether.

The cylinder 56 of the thermal short is constructed of a thin strip offinger stock formed into a cylindrical shape. The outer diameter of thecylinder is smaller than the inner diameter of the outer coaxial cableconductor 24, but the cylinder has expanding fingers which expand to thediameter of the outer conductor to assure good thermal contacttherewith. The bars 52 and 54 are straight, except for their inner ends72 and 74 which are curved to pass around the discontinuity between thecenter portion 66 and the expanded portion 68. The inner ends of thebars are joined to the center portions 66 of the conductor rod bysoldering, while the outer ends 76 and 78 of the bars are soldered tothe inside of the cylinder 56.

The distance D between the inner end 72 or 74 and the closest edge ofthe cylinder 56 is approximately equal to a quarter Weavelength of theWaves to be carried through the coaxial line. The effect of the bars 52and 54 which electrically connect the center and outer conductors butextend approximately a quarter wavelength along the transmission linebetween them, is similar to that of a quarter wavelength stub. A stub isa shortcircuited section of a coaxial line tied across the normaltransmission line, which can be used to support a center conductor of acoaxial line, but which acts as an insulator, this being well known inthe microwave art. A quarter wavelength stub could be used in place ofthe thermal short of the present invention; however, it would extendoutside of the outer conductor and prevent the removal of the length ofcoaxial cable through the opening in the housing 18 of the refrigeratorstructure. It is important to provide for the facile removal of thecoaxial cable inasmuch as leaks can develop in the vacuum seal 40, andthese leaks can be most readily corrected by replacing the entirecoaxial line. A coaxial line without any side projections can be readilyinserted through the housing 18 in the refrigerator structure. Thevacuum seal can also be replaced by replacement of only the centerconductor. This can be readily done because the thermal short readilyslides through the outer conductor 24.

The design of the thermal short is made to com ly with the followingconditions: The voltage standing wave ratio (VSWR) must be kept to aminimum, the impedance of the line must not be substantially affected,and heat transfer characteristics must be good. An increased VSWRresults in power loss and decreased signal-to-noise ratio. A change inimpedance results in mismatching between the coaxial line and thewaveguide or the maser input 14. In many types of installations, a 50ohm line is utilized, and it is important to keep the impedance of thecoaxial line at 50 ohms, to prevent losses due to mismatching. Theserequirements are important only for the bandwidth :at which thetransmission line is to be used, such as between 2.2 K. mHz. and 2.4 K.mHz. It might be expected that the length D of the shorting bars wouldbe a quarter wavelength, but it has been found in practice that thelength D should be slightly less than a quarter wavelength. For theconfiguration of a thermal short shown in the figures, a length Dapproximately 98% of a quarter wavelength of the center frequency of theband of frequencies to be transmitted has been found to be mostsatisfactory.

The expanded portion 68 has been found to be desirable in reducing VSWRand preventing a change of impedance. A person skilled in the microwaveart would probably expect that the shorting bars 52 and 54 wouldincrease the capacitance between the center and outer conductors 26 and24 and therefore a recessed portion, instead of an expanded portion,would be required, to correct for the increased capacitance. However, ithas been found that an expanded portion such as that shown at 68,instead of a recessed portion is necessary. In the configuration of thethermal short shown in the figures, an expanded portion 68 having adiameter approximately 25% larger than the outer diameter of the centerconductor 26 provides optimum results.

A thermal short has been constructed in accordance with the foregoingfor a A; inch nominal outside diameter coaxial line with a 0.341 inchouter diameter center conductor. The center and outer conductors wereconstructed of thin wall stainless steel with the inner surface of theouter conductor and the outer surface of the center conductor copperplated to carry radio frequency currents, and including a thin goldplating over the copper to reduce corrosion. The conductor rod 62 wasconstructed of a solid cylinder of copper with a center portion 66 of0.341 inch diameter, equal to the outer diameter of the center conductor26, and an expanded portion 68 of 0.430 inch diameter. Shorting bars 52and 54 of 0.079 inch diameter copper rods were used. For a frequencyband of transmission of 2.2 K. mHz. to 2.4 K. mHz., wherein a quarterwavelength of the center frequency is approximately 1.28 inches adistance D of 1.25 inches was found to be optimum. The cylinder 56 wasconstructed of a strip of berillium copper finger stock of half inchwidth, formed in a circle and with the ends overlapped, spot welded, andsoldered together. This design was utilized to obtain good impedancematch with the waveguide and maser, low VSWR in the signal frequencyrange, and sufficient thermal conductivity to transfer approximately 400milliwatts with a minimum temperature difference across the short. Theparticular dimensions of the thermal short were arrived at largely byexperiment, by trying various dimensions and testing the characteristicsof the resulting thermal short. The foregoing thermal short was found toprovide an insertion loss of 0.011 D a contribution to maser noisetemperature when cooled to K. of less than 0.1 K., and a measured VSWRof 1.05/1 at 2.3 K. mHz. The VSWR was less than 1.10/1 throughout arange of 2.75 K. mHz.

While particular embodiments of the invention have been illustrated anddescribed, it should be understood that many modifications andvariations may be resorted to by those skilled in the art, and the scopeof the invention is limited only by a just interpretation of thefollowing claims.

I claim:

1. A cooling device for use in combination with a transmission linehaving an inner electrical conductor, and an outer electrical conductordisposed about said inner conductor, said device comprising:

a thermally conductive body having a first end portion adapted tocontact said inner conductor, a second end portion adapted to contactsaid outer conductor, and a middle portion extending between said firstand second end portions adapted to extend primarily along the length ofsaid transmission line.

' 2. A cooling device as defined in claim 1 including:

means connected to said transmission line for delivering high frequencyelectromagnetic waves thereto within a predetermined band ofwavelengths; and wherein the extension of said middle portion of saidthermally conductive body between said first end portion and an areanear said second end portion, along the length of said transmissionline, is approximately equal to a quarter wavelength of the centerwavelength of said predetermined band of wavelengths.

. A cooling device as defined in claim 1 including:

a conductor rod adapted for connection in line with said inner conductorof said transmission line, and to extend along the length of saidtransmission line adjacent to said thermally conductive body, said conductor rod having an outer diameter greater than the outer diameter ofsaid inner conductor.

4. A cooled electrical conductor comprising:

a transmission line having an inner electrical conductor and an outerelectrical conductor disposed about said inner conductor;

conductor means having good thermal conductivity, including a first endportion in thermal contact with said inner conductor and a second endportion in thermal contact with said outer conductor, and a middleportion extending between said first and second end portions, saidmiddle portion extending primarily along the length of said transmissionline.

5. A cooled electrical conductor as defined in claim 4 including:

means connected to said transmission line for delivering high frequencyelectromagnetic waves thereto within a predetermined band ofwavelengths; and wherein the extension of said center portion of saidconductor means between said first end and an area near said second endadjacent to said outer conductor, along the length of said transmissionline, is approximately equal to a quarter wavelength of the centerwavelength of said band of wavelengths. v 6. A cooled electricalconductor as defined in claim 4 wherein:

said inner electrical conductor has a cylindrical exterior ofpredetermined outer diameter along a majority of its length, and has anenlarged inner conductor portion with a diameter larger than saidpredetermined outer diameter extending along the length of saidtransmission line for at least a portion of the extension along saidtransmission line of said thermal conductor means. 7. A cooledelectrical conductor as defined in claim 6 wherein:

said enlarged inner conductor portion extends along a majority of theextension along said transmission line of said thermal conductor means.8. A cooled electrical conductor as defined in claim 4 wherein:

said outer conductor has an inner surface of substantially circularcross section; and said second end of said conductor means comprises athin walled cylinder having a multiplicity of fingers which are springbiased against the inner surface of said outer conductor. 9. A cooledelectrical conductor as defined in claim 8 including:

means connected to said transmission line for delivering microwavesignals thereto primarily within a limited band of frequencies about apredetermined wavelength; and wherein the extension along the length ofsaid transmission line of said thermal conductor means between saidfirst end and the edge of said cylinder closet to said first end of saidconductor means is approximately equal to a quarter wavelength of saidpredetermined Wavelength. 10. A cooled electrical conductor as definedin claim 8 including:

means connected to said transmission line for carrying microwaves of afrequency within a predetermined frequency band, with a predeterminedcenter frequency of predetermined wavelength; and wherein said extensionalong said transmission line of said conductor means between said firstend of said thermal conductor means and said edge of said cylinderclosets to said first end is less than one quarter of said predeterminedWavelength.

11. A cooled electrical conductor as defined in claim 4 including:

a refrigeration station disposed about said outer conductor and inthermal contact therewith at a predetermined area along the length ofsaid transmission line; and wherein said second end of said thermalconductor means is disposed against said outer conductor at an areaadjacent to said refrigeration station.

12. A cooled electrical conductor as defined in claim 4 wherein:

said transmission line includes a first end portion surrounded by anenvironment at an ambient temperature normally greater than 270 K., asecond end portion normally at a temperature no more than severaldegrees Kelvin above absolute zero, and an intermediate refrigerationstation at a point between said first and second ends maintained at atemperature between the temperatures at said first and second ends; andwherein said second end of said thermal conductor means is locatedadjacent to said intermediate refrigeration station.

13. In a coaxial line for carrying microwaves from a source whichnormally delivers microwaves within a predetermined limited band offrequencies, the improvement comprising:

shorting means of thermally conductive material extending between theinner and outer conductors of said coaxial line, and having a first endportion disposed adjacent to said inner conductor and a second endportion disposed adjacent to said outer conductor of said coaxial line;

means thermally connecting said first end portion of said shorting meansto said inner conductor; and

means thermally connecting said second end portion of said shortingmeans to said outer conductor;

said shorting means extending along the length of said transmission linea distance approximately equal to a quarter wavelength of the centralfrequency of said band of frequencies.

14. The improvement in a coaxial transmission line as defined in claim13 wherein:

said means thermally connecting said second end of said shorting meansto said outer conductor comprises a cylinder disposed within said outerconductor, having a plurality of contact fingers in contact with theinner surface of said outer conductor.

15. The improvement in a transmission line as defined in claim 13including:

an expanded portion means disposed about said inner conductor along amajority of the extension of said shorting means along the length ofsaid transmission line, for enlarging the elfective outer diameter ofsaid inner conductor.

16. The improvement in a transmission line as defined in claim 15wherein:

the ratio between the outer diameter of said expanded portion and theouter diameter of said inner conductor at areas other than said expandedportion is approximately one and one-quarter.

17. A thermal short for a coaxial transmission line having a centralconductor and an outer conductor disposed about said central conductor,comprising:

a conductor rod having ends for thermal and electrical connection tosaid central conductor of said coaxial line and in line therewith, saidconductor rod having an expanded portion with an outer diameter greaterthan the outer diameter of said central conductor;

a thin-walled cylinder having a plurality of contact fingers for makingthermal contact with the inner surof said shorting bar is approximatelyequal to a quar face of said outer conductor of said coaxial line; andter Wavelength of the waves of said predetermined at least one shortingbar of thermally conductive mafrequency While passing through saidtransmission terial having a first end thermally and electricallyconline.

nected to said conducting rod at an edge of said ex- References Citedpanded portion and a second end thermally and elec- UNITED STATESPATENTS trically connected to the inside of said cylinder. 18. A thermalshort as defined in claim 17 including: 2421137 5/1947 Wheeler 333-96means connected to said transmission line for propagat- H, K SAALBACH, Pi E i ing microwaves approximately equal to a predcter- 10 minedfrequency; and wherein L. ALLAHUT, Assistant Examiner the distancebetween said first end of said shorting bar US. Cl. X.R.

and the edge of said cylinder closest to said first end 174-15; 33399

